Method for determining imaging sensor reference image

ABSTRACT

The present invention relates to a method for determining the reference image of an image sensor comprising a matrix of detector pixels.  
     The method comprises the acquisition of a black image I N  and two images I 1  and I 2  acquired in linear detection zones of the sensor.  
     The reference image is written:  
           I     R                 1       =       (         R   m     ×     I   1       -     I   2       )     /     (       R   m     -   1     )                                with                 Rm     =       1   N            ∑     i   ,   j                  I   2          (     i   ,   j     )       -       I   N          (     i   ,   j     )               I   1          (     i   ,   j     )       -       I   N          (     i   ,   j     )                 ,                 
 
     wherein I k (i,j) represents the value of the pixel of the image I K  detected by the pixel detector situated at the intersection of the row i and the column j of the detector pixel matrix, and N represents the total number of pixels of the matrix.

TECHNICAL FIELD AND PRIOR ART

[0001] This invention relates to a method for determining the referenceimage of an image sensor.

[0002] This invention also relates to a method for correcting imagedefects that implements a method for determining the reference imageaccording to the invention.

[0003] The invention applies to any field using digital images producedon the basis of x or γ radiation and, more particularly, in the field ofmedical imaging, where the images are formed using detectors utilizing aCCD (charge coupled device) camera or using gamma cameras integrating asensor in the form of a pixel array based on a CdZnTe or CdTesemiconductor material.

[0004] In digital medical imaging using x or γ rays, the imageacquisition system delivers an image representative of the quantity of xor γ photons transmitted through the radiographed object placed betweenthe source of x or γ rays and a detector. It is also possible for theobject itself to be the source of the γ rays.

[0005] An image sensor comprises a matrix of detector pixels and chargereading circuits. Generally, the same charge reading circuit allowsreading of the pixels of the same column of detector pixels. The imageobtained by an image sensor can contain a certain number of defects dueto sensor errors. First, the detector pixel matrix can contain defectpixels. These defect pixels can be isolated or grouped by row and/or bycolumn. It is also possible that the reading circuits and/or theconnections between pixels and reading circuits may be defective. In thelatter case, the image associated with an entire column of pixels isdefective.

[0006] Different processing methods are known for correcting imagesensor defects. A first treatment is based on image correction by blackimage. A second process relates to a correction to gain.

[0007] The correction by black image consists of subtracting from theimage to be corrected an image detected by the sensor in the absence oflight, commonly known as a black image. The gain correction is doneusing the acquisition of an image by uniform lighting of the camera.

[0008] Image correction can also be done by combining black imagecorrection and gain correction. The corrected image I_(c) can then bewritten:

I _(c)=(I _(NC) −I _(N))/(I_(G) −I _(N))  (1)

[0009] wherein I_(NC) is the uncorrected image, I_(N) is the black imageand I_(G) is the image obtained with uniform lighting of the camera.

[0010] Black image correction and gain correction of the image sensordefects are well known to the specialist in the art. Black imagecorrection is necessary in virtue of the presence of dark current andthe digital offset voltage. Likewise, gain correction is necessary invirtue of the variation in gain between the different pixels of the samematrix.

[0011] According to the state of the art, defect pixels do not provideany intrinsically usable information in respect of the detected image.They are corrected by calculation of a value of the gray level on thebasis of adjacent pixels by interpolation, for example.

[0012] The correction methods of the known prior art are based on thehypothesis that the response of the detectors to the radiation receivedis linear. This hypothesis is not always correct. In particular, theapplicant has found that this hypothesis is not valid for low graylevels.

[0013] The effect of non-linear charge detection for low gray levels isrepresented in FIG. 1.

[0014]FIG. 1 represents two curves of pixel gray levels (G) as afunction of the intensity of radiation (R) which illuminates a pixel. Afirst curve C1 represents the gray level of an idealized pixel and asecond curve C2 represents the gray level of a real pixel with loss ofcharge.

[0015] In curve C1, it appears that the illumination level increaseslinearly as a function of the intensity of the radiation, whatever thelevel of the radiation intensity.

[0016] In curve C2, in contrast, the gray level of the pixel increaseslinearly as a function of the radiation intensity only beyond a certainthreshold G₀. In an initial time, the increase of the gray level of thepixel does not increase linearly with the level of illumination due toan increasing loss of electrical charge in proportion to the increase ofillumination. In a second time, this loss reaches a maximum level andthe level of illumination then becomes linear relative to theillumination level.

[0017] The effect of these charge losses causes defects to appear on theimage. The charge losses can vary from one row to another and/or fromone column to another, the different behaviors then appearing betweenadjacent rows and/or columns. Furthermore, as was mentioned above,defective columns can also appear on the image in the case where thereading circuit of the pixels of one column and/or the connectionbetween the reading circuit and the pixels of the column are defective.

[0018] Correction of these defects is not possible by conventionalmeans. In particular, in the case where the columns are defective, theresult can be that a plurality of adjacent columns may be defective.Accordingly, it is impossible to correct one defective column using anadjacent valid column due to the fact of the absence of an adjacentvalid column for making a correction by interpolation.

SPECIFICATION OF THE INVENTION

[0019] The invention has none of the aforesaid drawbacks.

[0020] In fact, the invention relates to a method for determining thereference image of an image sensor comprising a matrix of detectorpixels, said method comprising the step of acquisition of a black imageI_(N). The method further comprises:

[0021] an acquisition step, wherein a first image I₁ is acquired by theaction of a first radiation, for which the sensor functions in a linearzone of detection;

[0022] an acquisition step, wherein a second image I₂ is acquired by theaction of a second radiation of an intensity greater than that of thefirst radiation and for which the sensor functions in a linear zone ofdetection;

[0023] a calculation step, wherein a mean value R_(m) is calculated,such that: $\begin{matrix}{{Rm} = {\frac{1}{N}{\sum\limits_{i,j}\frac{{I_{2}\left( {i,j} \right)} - {I_{N}\left( {i,j} \right)}}{{I_{1}\left( {i,j} \right)} - {I_{N}\left( {i,j} \right)}}}}} & (2)\end{matrix}$

[0024] wherein I_(k)(i,j) represents the value of the pixel of the imageI_(K) detected by the pixel detector situated at the intersection of therow i and the column j of the detector pixel matrix, and N representsthe total number of pixels of the matrix;

[0025] a calculation step, wherein a first reference image I_(R1) iscalculated such that the first reference image is written as:

I _(R1)=(R_(m) ×I ₁ −I ₂)/(R_(m)−1)  (3)

[0026] The invention also relates to an improvement of the aforesaidmethod. According to the improvement of the invention, the methodcomprises the following additional steps:

[0027] calculation of a black difference image D_(N) such that:

D _(N) =I _(R1) −I _(N)  (4)

[0028] summation of the system of pixel values of each column of theblack difference image;

[0029] division of each sum obtained at the end of the summation step bythe number of pixels contained in the column so as to obtain a meanpixel value for each column;

[0030] creation of a smoothed image I_(EP) by applying to the differentpixels of each column the mean pixel value obtained in the aforegoingstep for said column;

[0031] calculation of a second reference image I_(R2) such that:

I _(R2) =I _(N) −I _(EP)  (5).

[0032] The invention also relates to a method for correcting the imagedefects in images obtained using an image sensor, characterized in thatit implements a method for determining the reference image of the imagesensor according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Other features and advantages of the invention will becomeapparent when reading the description of a preferred embodiment withreference to the appended figures, wherein

[0034]FIG. 1 represents the pixel gray level curves as a function of theradiation intensity illuminating the pixel;

[0035]FIG. 2 represents a flow chart of the method for determining thereference image according to the invention;

[0036]FIG. 3 represents a flow chart of an improvement of the method fordetermining the reference image according to the invention;

[0037] On all the figures, the same reference numerals denote the samecomponents.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

[0038]FIG. 1 was described previously and repetition would serve nouseful purpose.

[0039]FIG. 2 represents a flow chart of the method for determining areference image according to the invention.

[0040] The method comprises firstly three steps for acquiring imagesusing an image sensor. A first step 1 is the image acquisition step foracquiring a black image I_(N). A second step 2 is an acquisition stepfor acquiring a first image I₁ by the operation of a first radiation anda third step 3 is an acquisition step for acquiring a second image I₂ bythe operation of a second radiation. The illumination conditions of thesensor for acquiring the images I₁ and I₂ are done in the linear zone ofoperation of the sensor. Image 12 is obtained for an incident radiationthat is greater than the radiation intensity that is used for obtainingthe image I₁.

[0041] The radiation intensities for acquiring the images I₁ and I₂ canbe chosen in different ways. For example, it is possible to choose, foracquiring image I₂, an incident radiation that generates a gray levelapproximating the maximum level that it is possible to reach beforesaturation of the sensor and, for acquiring the image I₁, an incidentradiation of half the intensity of the radiation that enables acquiringthe image I₂. This selection method is possible only if the image sensorhas a linear behavior in the half-height of the gray levels, which isusually the case.

[0042] Another way for acquiring the images I₁ and I₂ will now bedescribed.

[0043] Firstly, two initial images are acquired I₁₀ and I₂₀ at twodifferent radiation intensities, the radiation intensity for acquiringthe image I₂₀ being greater than the radiation intensity for acquiringthe image I₁₀. Then a projection of these two images is done in thedirection of the columns. The projections obtained each form acontinuous profile that can include peaks representative of the defectpixel columns. If the peaks are identical from one profile to another,it is deduced that the two initial images I₁₀ and I₂₀ have been taken ina zone of linear behavior of the sensor. The image I₁₀ is then chosen asthe first image I₁ and the image I₂₀ as the second image I₂.

[0044] If the peaks are not identical from one profile to the other, itis deduced that it is not in a zone of linear behavior of the sensor.The linear behavior zone of the sensor being situated towards theincreasing intensities (cf. FIG. 1), a new initial image is thusobtained for replacing the initial image I₁₀ previously acquired withthe lowest intensity and the projection step is repeated. If the newinitial image I₁₀ and the initial image I₂₀ previously acquired haveidentical continuous profiles, they are then chosen respectively as thefirst image I₁ and second image I₂. If not, new image acquisitions arecarried out until obtaining identical profiles enabling selection of thefirst and second images.

[0045] Each image I₁, I₂ can be a single image obtained consequent to asingle illumination of the sensor or an averaged image obtainedconsequent to a plurality of successive illuminations of the sensorunder substantially identical conditions.

[0046] Steps 1, 2 and 3 are followed by a calculation step 4 forcalculating a mean ratio R_(m) defined by the equation below:$\begin{matrix}{{{Rm} = {\frac{1}{N}{\sum\limits_{i,j}\frac{{I_{2}\left( {i,j} \right)} - {I_{N}\left( {i,j} \right)}}{{I_{1}\left( {i,j} \right)} - {I_{N}\left( {i,j} \right)}}}}},} & (6)\end{matrix}$

[0047] wherein I_(k)(i,j) represents the value of the pixel of the imageI_(K) detected by the pixel detector situated at the intersection of rowi and column j of the detector pixel matrix, and N represents the totalnumber of pixels of the matrix.

[0048] A first reference image I_(R1) can then be calculated using astep 5 that follows step 4. Where:

I _(R1)=(R _(m) ×I ₁ −I ₂)/(R _(m)−1)  (7).

[0049] The quantity I_(R1) can then be used as the reference image in animage defect correction procedure according to the invention, forexample, by replacing the quantity I_(N) in the equation (2).Advantageously, the reference image I_(R1) enables correcting theaforesaid non-linear effects.

[0050]FIG. 3 represents an improvement of the method for determining thereference image according to the invention.

[0051] According to the improvement of the method of the invention, theimage I_(R1) is not directly used as reference image but serves todefine another reference image I_(R2) different from the image I_(R1).As will become apparent in the course of the description, the referenceimage I_(R2) is particularly well adapted for correcting defect pixelsthat are present in the form of columns of defect pixels. A plurality ofsteps then follow the aforementioned step 5.

[0052] Step 6, which immediately follows step 5 is a black differenceimage D_(N) calculation step between the image I_(R1) and the blackimage I_(N). Where:

D _(N) =I _(R1) −I _(N)  (8).

[0053] A step 7 follows step 6 and is a summation of all of the pixelvalues of each column of the black difference image D_(N), then a step 8of division of each sum so obtained by the number of pixels of eachcolumn so as to obtain a mean pixel value M_(j) for each column j of theblack difference image. A step 9 of smoothing follows step 8. Step 9enables creating a smoothed image I_(EP) obtained by applying to thedifferent pixels of a column j the average value M_(j) of the columncalculated previously. The reference image I_(R2) using the improvementof the invention is then obtained by subtraction of the smoothed imageI_(EP) from the black image I_(N). Where:

I _(R2) =I _(N) −I _(EP)  (9).

[0054] In addition to the correction of the aforementioned non-lineareffects, the reference image I_(R2) thus advantageously enables takinginto account the detection defects that may appear in the entirecolumns.

1. A method for determining the reference image of an image sensorcomprising a matrix of pixel detectors, the method comprising anacquisition step for acquiring a black image I_(N), characterized inthat it comprises, in addition: an acquisition step for acquiring afirst image I₁ by the operation of a first radiation for which thesensor functions in a linear detection zone; an acquisition step foracquiring a second image I₂ by the operation of a second radiation of anintensity greater than that of the first radiation and for which thesensor functions in a linear detection zone; a calculation step forcalculating a mean value R_(m), such that:${{Rm} = {\frac{1}{N}{\sum\limits_{i,j}\frac{{I_{2}\left( {i,j} \right)} - {I_{N}\left( {i,j} \right)}}{{I_{1}\left( {i,j} \right)} - {I_{N}\left( {i,j} \right)}}}}},$

wherein I_(k)(i,j) represents the value of the pixel of the image I_(K)detected by the pixel detector situated at the intersection of the row iand the column j of the detector pixel matrix, and N represents thetotal number of pixels of the matrix; a calculation step, wherein afirst reference image I_(R1) is calculated such that the first referenceimage is written as: I _(R1)=(R _(m) ×I ₁ −I ₂)/(R _(m)−1).
 2. Themethod according to claim 1, characterized in that it comprises thefollowing additional steps: calculation of a black difference image DNsuch: that: D _(N) =I _(R1) −I _(N); summation of the set of pixelvalues of each column of the black difference image; division of eachsum obtained at the end of the summation step by the number of pixelscontained in the column so as to obtain a mean pixel value for eachcolumn; creation of a smoothed image I_(EP) by applying to the differentpixels of each column the mean pixel value obtained in the aforegoingstep for said column; calculation of a second reference image I_(R2)such that: I _(R2) =I _(N) −I _(EP).
 3. The method according to claim 1or 2, characterized in that the intensity of the second radiation issubstantially double the intensity of the first radiation.
 4. The methodaccording to claim 3, characterized in that the intensity of the secondradiation has a value substantially equal to an intensity of radiationthat generates a gray level approximating the maximum level that ispossible to reach before saturation of the sensor.
 5. The methodaccording to any one of claims 1 or 2, characterized in that the firstimage I₁ and the second image I₂ are chosen in the following way: twoinitial images are acquired I₁₀ and I₂₀ at two different radiationintensities, the radiation intensity for acquiring the image I₂₀ beinggreater than the radiation intensity for acquiring the image I₁₀;projection of these two initial images I₁₀ and I₂₀ is done in thedirection of the columns; the projections obtained each form acontinuous profile that can include peaks representative of the detectordefect pixels; comparison of the continuous profiles, and if the peaksare identical from one profile to another, image I₁₀ is then chosen asthe first image I₁ and the image I₂₀ as the second image I₂, or if thepeaks are not identical from one profile to the other, acquisition of atleast one new initial image for replacing at least the initial image I₁₀and a new projection step; acquisition of at least one new initial imageand the new projection step being carried out until obtaining identicalcontinuous profiles enabling selection of said first and second images.6. The method for correcting image defects obtained using an imagesensor, characterized in that it implements a method for determinationof the reference image of an image sensor according to any one of claims1 to 5.